1. Field of the Invention
This invention relates to an improved method and apparatus for obtaining the transmission spectra of liquid, paste and solid samples, and in particular to sample holders. It is applicable over a wide spectral range, but especially in the mid infrared region where absorption coefficients are often very high and corresponding sample thicknesses need to be very small, typically of the order of 10 microns for pure substances or simple mixtures.
2. Discussion of the Prior Art
There are a number of methods known for supporting samples for infrared spectroscopy. For powdered solids the sample is normally held within pressed discs of infrared transmitting material, typically KBr. The disadvantage of this method is that this has only a limited spectral range due to the absorption of KBr.
Spaced sample cells of infrared transmitting material are also used to support liquid samples. However, these typically have a limited spectral range. Other disadvantages are difficulty of construction and use, cost, lack of chemical inertness and limited spectral range.
Another known method for supporting liquid samples involves the use of thin films of porous material. Such discs can be obtained from 3M, and consist of Teflon or polythene. Teflon and polythene have absorption bands, but in different regions of the spectrum. Therefore it is possible to cover a wide spectral range by the selective use of the two materials. However, it is inconvenient to use different materials for different spectral regions and the absorption bands again reduce the quality of the recorded spectra.
Another method for supporting liquid samples is the use of coarse gauzes which take up liquid by capillary action. Janos Technology Inc produce screens formed by such a gauze (ECRAN (RTM) Screen Cells), which are described in U.S. Pat. No. 5,453,252. The gauze is formed from polymer coated glass fibres, onto which a liquid or powdered solid is placed. The gauze is approximately 300 microns thick where the fibres cross each other, and the holes between the fibres are approximately 1 mm across. Solid samples can also by supported by such screens.
The disadvantage of the above screens is their thickness and hole size. For many samples a thickness of 10-20 microns is preferable, especially in the mid infrared region, and the thickness of 300 microns provided by the Ecran Screen Cells prevents samples being thin enough. There is also the problem that with hole sizes in the region of 1 mm, the surface tension of many liquids is not sufficient to hold the liquid in place across the holes. Solid samples may be introduced by covering the mesh with a solution of the sample in a suitable solvent and allowing the solvent to evaporate. The large hole size of the Ecran screen cell makes it extremely difficult to achieve a continuous film by this means.